Is My FuelPump Failing?

PonyGTrider

Active Member
Feb 27, 2019
173
32
38
Mexico
Hi all,
My ‘90 GT 306 has been running great until two days ago. I started it in my garage put it in reverse and as I released the clutch pedal it just died. Tried to re-start it but just span without firing up.

I the started paying attention and the fuel pump was dead quiet zero priming. Yesterday I kept trying to hear that pump without any response. I have a 190 lph pump.
I started checking the system and found no problems:
* Fuel Inertia Switch good (No tripped)
* Fuel Pump Relay good
* EEC Relay good
* Turned key to ON and heard the fuel pump relay actuating.
* Left the ign. Switch ON jumped the fuel pump at the diagnostic port, I heard the EEC relay switching ON, zero priming. Went to the the rear and unplugged the fuel pump/sending unit connector and measures the voltage and had 12 volts. Connected the fuel pump to the battery and ran fine!
Thinking on a loose connection at The pump harness, I snugged the female terminals with a small screwdriver. Removed the jumper at the diagnostic port and turned the ignition switch OFF . Back to the rear and plugged the pump harness., turned the ign. Switch ON and bingo!!! The pump is priming!!!!! Did it a few times and worked fine!!!
My question is: What the heck was wrong? Did I have just a loose connection at the pump harness, or was the pump stacked and just got free? Or is it my pump that is going bad???? Never had this problem before, and really don’t want to do the same failure while out in the road.
Any help and ir suggestions will be greatly appreciated, specially from anyone of you that had experienced the same situation. Sorry about the long post , just want to give all the details of what I’ve done.
Thank you!!!
 
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CAMTWO1070

Active Member
Dec 17, 2021
163
48
38
NY
Sometimes I would answer a solid yes to your question as it definately seems your pump is kaput but I had an issue similar to yours and it turned out after replacing the fuelpump and no dice and re-removing the tank to do a further diagnosis and go over my install was the bulkhead hatch top at the top of the tank where the wires for the fuelpump connect via brass crush-roll rivets on the inside....

One thing you didnt mention is if theres power to the wire going into the fueltank at the plug....

On my vehicle the rivet for the fuelpump power wasnt crushed down correctly and was loose enough to cause it to spark out....I cleaned the spot up and with a punch I tightened up on the connection and made it work again which didnt make me happy knowing the old pump was ok when I tested it on the bench or that the fueltank was supposed to be high quality for the $639.00 I paid for it with the whole installation kit....

If you do change the pump upgrade to a 255lph even though with BFSC and HP of your engine youll feed the need at around 160lph but with the extra fuel volume it will be way worth it as it will lower the fluxuating duty cycle correction the injectors have to make and some will try to debate this but if volume is increased and fuel injectors dont fluxuate as badly at lower cruising speeds like they do when running larger injectors so doesnt that give aid the injector duty cycle aswell when the injector fuel demand has no noticeable decrease in flow because of the overly increased volume which in essence keeps the injectors more linear and less redundant...?

If you notice on the schematic that the fuelpump power wire melds with the wire on pin #19 on the ECU plug.....That wire slightly increases / decreases fuelpump power to get it to spin faster and slower so if you ever get a #96 error code that says your fuelpump is operating without a voltage monitor...

96 - Fuel Pump Circuit Open-Battery To PCM (CM,O)

Back in the days the notrous guys used to wire the secondary wire to an in-line booster pump relay on the to boost fuel pressure when they added the Nitrous Express kits to simplify the installation...........LOL

88-91_5.0_EEC_Wiring_Diagram1.gif


The fuelpump secondary circuit is similar to this but not as complex as this one can be programmed.....I have one of these in my 8 second Junkyard Nova with 5.3 93mm turbo and 300shot of laughing gas...

s-l1600 (24).jpg


Another thing you can do is check the voltage on pin #19 and check for voltage at pin #22....If you get voltage at pin #22 theres an issue with the wiring or relay...

Oh and just because a relay seems to click and transfers power its the batteries amperage that gets lost through age and contact corrosion inside a relay so if the relays are older than 10years old replace them first and on the unibody there are ground wires ...clean them up as they are very important grounding wires for the ECU and the ECU relay that feeds the ECU and fuelpump relay........

Screenshot 2022-10-25 190227.jpg


Good Luck
 
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AeroCoupe

Founding Member
Oct 28, 2001
547
235
73
Jenks, OK
It’s a known issue with the fuel pump / sending unit wiring harness plug. They corrode up, get loose, and sometimes melt. Last time mine was out I tightened the connections and used some dielectric grease to seal it up to keep moisture out.

Pump failure is not as common as it seems as we only see threads on the ones that do vs the thousands that don’t. Most of the time it’s the fuel pump relay, jacked ECU, wiring issue, rotted rubber line between the pump and hanger, or some other bizarre and obscure issue but not the pump.

Don’t jump to a 255 lph pump if you don’t need it. Remember the unspent fuel is just going to be returned for the tank. If it’s a big enough volume it can really churn the fuel up around the suction if the pump at lower tank levels. There is a mod to fix this but just throwing it out there. Also, when you get up to the 255 lph size and larger you may want to look at an auxiliary relay at the rear of the car. The factory wiring is good for about 20A of current draw which the 255 is under but the higher the amp draw the voltage will fall which will reduce the pump output. Your call here as some do this and I would venture to say most don’t.

I am adding the relay to my Coupe this winter as the guy that tuned it said I’m getting up on the hp limit of the 255 that is in the car so it’s getting a 340.

Anyhow, I’d just run it and see how it goes as I’d say you fixed it.
 
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PonyGTrider

Active Member
Feb 27, 2019
173
32
38
Mexico
Sometimes I would answer a solid yes to your question as it definately seems your pump is kaput but I had an issue similar to yours and it turned out after replacing the fuelpump and no dice and re-removing the tank to do a further diagnosis and go over my install was the bulkhead hatch top at the top of the tank where the wires for the fuelpump connect via brass crush-roll rivets on the inside....

One thing you didnt mention is if theres power to the wire going into the fueltank at the plug....

On my vehicle the rivet for the fuelpump power wasnt crushed down correctly and was loose enough to cause it to spark out....I cleaned the spot up and with a punch I tightened up on the connection and made it work again which didnt make me happy knowing the old pump was ok when I tested it on the bench or that the fueltank was supposed to be high quality for the $639.00 I paid for it with the whole installation kit....

If you do change the pump upgrade to a 255lph even though with BFSC and HP of your engine youll feed the need at around 160lph but with the extra fuel volume it will be way worth it as it will lower the fluxuating duty cycle correction the injectors have to make and some will try to debate this but if volume is increased and fuel injectors dont starve a lil at full demand like they do with a stock fuelpump running larger injectors doesnt that give aid the injector duty cycle aswell when the injector fuel demand has no noticeable decrease in flow which in essence keeps the injectors more linear and less redundant...?

If you notice on the schematic that the fuelpump power wire melds with the wire on pin #19 on the ECU plug.....That wire slightly increases / decreases fuelpump power to get it to spin faster and slower so if you ever get a #96 error code that says your fuelpump is operating without a voltage monitor...

96 - Fuel Pump Circuit Open-Battery To PCM (CM,O)

88-91_5.0_EEC_Wiring_Diagram1.gif


Another thing you can do is check the voltage on pin #19 and check for voltage at pin #22....If you get voltage at pin #22 theres an issue with the wiring or relay...

Oh and just because a relay seems to click and transfers power its the batteries amperage that gets lost through age and contact corrosion inside a relay so if the relays are older than 10years old replace them first and on the unibody there are ground wires ...clean them up as they are very important grounding wires for the ECU and the ECU relay that feeds the ECU and fuelpump relay........

Screenshot 2022-10-25 190227.jpg


Good Luck
Thanks man for all your suggestions. I don’t know if the pins on top of the fuel tank are loose because I didn’t took the tank down.

I will definitely look on all of your ideas. This is the first time I’ve experienced this failure.
Thanks a lot
 

PonyGTrider

Active Member
Feb 27, 2019
173
32
38
Mexico
It’s a known issue with the fuel pump / sending unit wiring harness plug. They corrode up, get loose, and sometimes melt. Last time mine was out I tightened the connections and used some dielectric grease to seal it up to keep moisture out.

Pump failure is not as common as it seems as we only see threads on the ones that do vs the thousands that don’t. Most of the time it’s the fuel pump relay, jacked ECU, wiring issue, rotted rubber line between the pump and hanger, or some other bizarre and obscure issue but not the pump.

Don’t jump to a 255 lph pump if you don’t need it. Remember the unspent fuel is just going to be returned for the tank. If it’s a big enough volume it can really churn the fuel up around the suction if the pump at lower tank levels. There is a mod to fix this but just throwing it out there. Also, when you get up to the 255 lph size and larger you may want to look at an auxiliary relay at the rear of the car. The factory wiring is good for about 20A of current draw which the 255 is under but the higher the amp draw the voltage will fall which will reduce the pump output. Your call here as some do this and I would venture to say most don’t.

I am adding the relay to my Coupe this winter as the guy that tuned it said I’m getting up on the hp limit of the 255 that is in the car so it’s getting a 340.

Anyhow, I’d just run it and see how it goes as I’d say you fixed it.
Thank you for replying. Like I said I’ve never experienced this problem before. In any case I crimped the female sockets and I’m sure it shouldn’t get loose anymore. I’ll look into all the details you suggest to see any other possible downgrading on the wiring/relays and also will look into installing another pump relay with heavier gauge wires.

Thanks for your time.